The invention is related to a screw cap, in particular for fluid reservoirs of hydraulic actuating systems. The cap has an opening with a pressure-balancing device sealingly surrounding the opening. The pressure-balancing device is comprised of a protecting cover and a port in the protecting cover.
Known types of such screw caps are usually utilized in fluid reservoirs of hydraulic brake systems in automotive vehicles. One disadvantage of such caps occurs during a motor wash when cleansing liquid is splashed against the vehicle motor at high pressure. The cleansing liquid can penetrate into the interior space of the fluid reservoir through the pressure-balancing device.
Since the brake fluid is hygroscopic which means that it is capable of absorbing water, the boiling point of the brake fluid is undesirably reduced. The result thereof is that if that water reaches the wheel cylinder space of a brake, vapor locks come about at an early moment in the wheel cylinder space under the effect of elevated temperatures, so that considerable lost travel is caused at the brake pedal. The result may even be a total failure of a brake system.
Sealing of the pressure-balancing device to obtain absolute tightness is not possible because in that case an overpressure or a vacuum would be generated in the fluid reservoir on account of the volume of brake fluid required to be delivered to the wheel brakes. The consequence is that a smooth flow of the brake fluid would no longer be guaranteed.
The present invention has an object to provide an improved screw cap through which, under normal conditions or in the presence of extreme external influences, no liquid penetrates into a tank or reservoir.